This invention relates to a method and system for producing degasified purified water utilizing reverse osmosis. More particularly this invention relates to a method and system for purifying water by reverse osmosis followed by degasifying by indirect exposure of water to low pressure across a hydrophobic membrane.
At the present time a reverse osmosis step is used to remove impurities, primarily microorganisms, pyrogens and ionic material from water by utilizing a reverse osmosis membrane. Usually, reverse osmosis is conducted by contacting the incoming water tangentially to the membrane so that the waste water comprising the retentate washes the membrane to reduce accumulation of impurities retained on the membrane during use. The fraction of the total water which comprises the retentate varies depending on the quality of feed water, desired product quality and the type of membrane material employed in the reverse osmosis unit. In a common mode of operation, between about 80 and 90% of the incoming water to the reverse osmosis step washes the upstream surface of the reverse osmosis membrane and is removed as retentate or waste. The remaining portion of the incoming water passes through the reverse osmosis membrane as purified water. This mode of operation, wherein a large percentage of the water constitutes reject water, is necessary in order to prevent premature plugging of the reverse osmosis membrane.
In some modes of use, it is necessary to degasify purified water such as when the water is utilized in analytical apparatus for cleaning, dilution or in a mixture with reagents at elevated temperature, usually about 37.degree. C. As the temperature of the water is increased, some fraction of the gases which were soluble at the inlet temperature (usually &lt;37.degree. C.) become insoluble and form bubbles. These bubbles render the measurements taken with the analytical apparatus inaccurate due to refraction of ultraviolet and visible light typically used to make the measurements.
Presently, product water from a reverse osmosis unit can be degasified by a vacuum generated with a pump. The use of a pump is undesirable since it requires the use of expensive protection devices such as water traps, dessicators and vacuum regulators. In addition, the use of a vacuum pump is undesirable because of the cost of this additional component. In addition, should any of the protection devices fail, the pump will likely be ruined by water intrusion on its vacuum side.
It is disclosed in U.S. Pat. No. 4,391,713 to purify water with a reverse osmosis unit and to remove carbon dioxide which was deliberately generated from purified water by a vacuum generated in an eductor unit. The vacuum is generated by passing waste water from the reverse osmosis unit through the eductor. The purified water is directed to a storage tank having an open gas volume above the top surface of the stored purified water. The vacuum produced within the eductor is effected above the open gas volume. Water in the tank is recirculated and sprayed onto the top surface of the stored water so that the sprayed water releases its carbon dioxide. Filtered air is introduced into the open gas volume to sweep up the free carbon dioxide and to remove it under the reduced pressure generated by the eductor. This system is undesirable since it requires a noncontinuous batch process wherein only a volume of water equal to that of the storage tank can be treated at a given time. After treatment is complete, the treated water is removed and the tank is filled with a new volume of water. In addition, a means for providing a clean volume of sweeping gas is required which may introduce a new gas into the water. Accordingly, it would be desirable to provide a system for degasifying purified water which permits continuous processing of the water and which eliminates the need for a sweeping gas. In addition, it would be desirable to provide such a system which eliminates the need for expensive pumps.